1. Field of the Invention
The present invention generally relates to digital transmission systems, and more particularly relates to a system and method for reducing the effects of clipping in a DMT transceiver.
2. Discussion of the Related Art
In order to make high data rate interactive services such as video conferencing available to more residential and small business customers, high-speed data communication paths are required. Although fiber optic cable is the preferred transmission media for such high data rate services, it is not readily available in existing communications networks, and the expense of installing fiber optic cable is prohibitive. Current telephone wiring connections, which consist of copper twisted-pair media, are not designed to support the data rates, or bandwidth, required for interactive services. Asymmetric Digital Subscriber Lines (ADSL) technology has been developed to increase the effective bandwidth of existing twisted-pair connections, allowing interactive services to be provided without requiring the installation of new fiber optic cable.
Discrete Multi-Tone (DMT) is a multicarrier technique that divides the available bandwidth of twisted-pair connections into many subchannels. The DMT technique has been adopted by the ANSI T1E1.4 (ADSL) committee for use in ADSL systems. In ADSL, DMT is used to generate 250 separate 4.3125 kHz subchannels from 26 kHz to 1.1 MHz for downstream transmission to the enduser, and 26 subchannels from 26 kHz to 138 kHz for upstream transmission by the enduser. The transmission capability of the individual subchannels are evaluated for each connection, and data is allocated to the subchannels according to their transmission capabilities (the number of bits each subchannel can support). Subchannels that are not capable of supporting data transmission are not used, whereas the bit-carrying capacity of subchannels that can support transmission is maximized. Thus, by using DMT in an ADSL system, the transmission capability of each twisted-pair connection is maximized over the fixed bandwidth.
Once the transmission capability of a connection has been established, the data transfer process begins by encoding the data. Data in an ADSL system is grouped in frames, where a frame represents a time-slice of the data to be transmitted. Bits from the frames are assigned to the subchannels based on the number of bits that each subchannel can support, and the subchannels are encoded by creating a frequency-domain vector set. Frequency-domain vectors in the vector set use phase and magnitude components to encode the values of the bits. An Inverse Fast Fourier Transform (IFFT) performs a frequency-to-time conversion of the frequency-domain vectors, resulting in digital time-domain information. A digital-to-analog converter (DAC) then converts the digital information to an analog signal which a transmitter transmits onto the copper twisted-pair media. The ANSI T1E1.4 standard defines the average power requirement of the signal for transmission on the twisted pair media, and in order to satisfy the power requirement, an amplifier is required.
When the analog signal from the DAC overshoots a magnitude threshold, which is dependent on the power supply used in the system, clipping of the signal can occur. Peaks in the analog signal occur when the vectors in the frequency-domain vector set are combined through the IFFT. Each frequency-domain vector contributes to the magnitude of the time-domain signal, and if the frequency-domain vectors are such that their contributions are concentrated in one area of the time-domain signal, peaks can result. Clipping occurs when the Integrated Circuit (IC) on which the transmitter is fabricated cannot support the dynamic range requirements of the peaking signal and can result in the loss of information. Section 7.11.1 of the T1E1.4 standard addresses this problem and limits the information loss by specifying that the probability of the signal clipping be less than one in 10 million.
The probability of a peak exceeding the magnitude threshold (maximum signal power on the IC) is based on the Peak-to-Average Ratio (PAR) of the signal, which is a ratio of the maximum power of the signal to the average power of the signal. If the average power is small compared to the magnitude threshold, a large peak can occur without exceeding the point where clipping occurs. Therefore, one method of reducing the number of peaks exceeding the magnitude threshold for a fixed PAR is to reduce the average power of the signal. Although this reduces the occurrence of clipping, lower signal strength increases susceptibility to noise, which can cause other transmission problems. Another method of reducing the probability of clipping utilizes a larger power supply, which raises the magnitude threshold where clipping occurs. A larger power supply, however, increases cost and consumes excessive power and adds additional regulatory requirements.
Therefore, a need exists for a method and/or apparatus to reduce the occurrence of signal peaks in a DMT transmitter such that the power supply of the system can be reduced, the signal strength can be raised, and/or the probability of the signal clipping can be reduced.
Several approaches to address this problem have been made by systems known in the prior art. For example, U.S. Pat. No. 5,835,536 discloses one such system. As illustrated in FIG. 1, U.S. Pat. No. 5,835,536 discloses a system having a DMT transmitter including a symbol generator 104, a magnitude comparator 112, and a magnitude adjuster 114. The DMT transmitter receives framed data 102 at the symbol generator 104 and generates a time-domain DMT symbol 110 based on the framed data 102. In an ADSL system, the symbol generator 104 includes an ADSL constellation encoder 106 and an IFFT block 108. The ADSL constellation encoder 106 encodes the framed data 102 by mapping the values of the data bits to frequency-domain vectors on subchannels within the bandwidth used for ADSL transmission. The number of bits that can be encoded on each subchannel may be determined by sending a training signal. The IFFT block 108 transforms the frequency-domain vectors to the time-domain, resulting in a time-domain DMT symbol 110.
The magnitude comparator 112 compares the magnitude of the time-domain DMT symbol 110 to a magnitude threshold to determine if clipping will occur. The magnitude adjuster 114 includes a magnitude adjusting symbol 116, a multiplexer or mux 118, and an adder 120. When the magnitude comparator 112 determines that the magnitude of the time-domain DMT symbol 110 is such that clipping will occur, it directs the mux 118 to pass the magnitude adjusting symbol 116 to the adder 120 which adds it to the time-domain DMT symbol 110 such that magnitude of the time-domain DMT symbol 110 is reduced, effectively reducing the PAR of the system.
Such a system, however, always makes the same magnitude of adjustment, regardless of how much the magnitude exceeds the clipping threshold.
As illustrated in FIG. 2, U.S. Pat. No. 5,835,536 also discloses an alternative DMT transmitter which includes a symbol generator 204, a magnitude comparator 210, and a Symbol modifier 208. The symbol generator 204 generates a time-domain DMT symbol 206 based on the framed data 202. The magnitude comparator 210 compares the magnitude of the time-domain DMT symbol 206 to a magnitude threshold to determine if clipping will occur. When the magnitude of the time-domain DMT symbol 206 compares unfavorably to the magnitude threshold, the symbol modifier 208 modifies the time-domain DMT symbol 206 to produce a modified time-domain DMT symbol 212 of reduced magnitude. The symbol modifier 208 may modify the symbol by altering the mapping function used for encoding the data, altering certain vectors in the frequency-domain representation of the DMT symbol, etc. The symbol modifier 208 may also produce a modification signal 207, wherein the modification signal 207 characterizes the modified time-domain DMT symbol 212.
U.S. Pat. No. 5,623,513 discloses a prior art system for mitigating the effects of clipping and quantization in a digital transmission system. Such a system is illustrated in FIG. 3, which illustrates an implementation of a clipping or truncation function 328 prior to a sampling point for an echo canceler. In the system of FIG. 3, the signal which is supplied to the D/A converter 314, having been limited and truncated in the unit 328 to take into account the characteristics of the DAC, is not subject to any further quantization noise or clipping noise within the DAC. This same signal is supplied to the echo canceller 326, which accordingly operates on the same signal which is supplied to the DAC and which is not subject to further nonlinear distortion due to quantization or clipping.
While such a system reduces the effects of quantization and clipping, insofar as the echo canceler is concerned, effects of this clipping are still noted as distortion in a remote receiver.
Accordingly it is desired to provide a DMT transmitter that avoids clipping altogether.
Certain objects, advantages and novel features of the invention will be set forth in part in the description that follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned with the practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the objects and advantages of the present invention, the present invention is directed to a system and method for reducing the need to perform signal clipping in a DMT transmitter. In accordance with one aspect of the invention, a method performs an inverse Fourier Transform on the input to produce a time-domain, digital value to be transmitted to a remote receiver. The method then evaluates the magnitude of the digital value to determine whether the magnitude exceeds a threshold value. Then, the method alters the input and re-performs an inverse Fourier Transform on the altered input, only if the step of evaluating the magnitude determines that the magnitude of the digital value exceeds the threshold value.
In this way, the inverse Fourier Transform is repeated (recursively) until a digital, time-domain signal is produced that is below a threshold level that would otherwise be clipped. Thus, no distortion is introduced into the signal due to clipping, either at the remote receiver or as echo at the near end receiver.
In accordance with another aspect of the invention, a discrete multi-tone (DMT) transmitter circuit is provided, which reduces the need to perform signal clipping. The circuit includes IFFT means for generating an inverse Fourier Transform, and generating means for generating in input for the IFFT means. Preferably, the input is generated from a mapper circuit, which maps a signal value into a signal constellation, to produce an encoded symbol preferably having good noise immunity for communication to a remote receiver. The circuit further includes determining means for determining whether an output of the IFFT means exceeds a threshold value. This determining means may be in the form of a magnitude detector circuit that is configured to detect a digital value having a magnitude that exceeds a predetermined maximum value (e.g., a clipping threshold value). Finally, the circuit further includes altering means for altering the input if the output of the IFFT means exceeds the threshold value. In one embodiment, this altering means could be in the form of circuit that is configured to rotate the predefined signal constellation of the mapper circuit. In another embodiment, the altering means could be operative to add additional input values to otherwise unused bins for the IFFT. Adding values in this way will, necessarily, alter the output of the IFFT, and therefore affect the magnitude of the signal output therefrom.
In addition to recursively altering the input until the magnitude of the digital signal output from the IFFT is such that no clipping will be performed on the signal, one embodiment of the invention may further ensure that only a few input bits have been altered. Specifically, the input may be recursively altered until no clipping will result, and only a relatively few input bits have been altered. With the alteration of only a few bits, a Reed Solomon decoder at the remote receiver may treat the alteration as an error, and correct the error (e.g., error correction coding). In such an embodiment, information about the manner in which the input was altered need not be communicated to the receiver.